recessed outdoor mounting boxes for electric and hose bib

How do you install these pvc boxes for a hose faucet with existing siding. Im having problems visually figuring out how the heck you get the back plate fastened to the plywood behind the siding without cutting a huge box out…

You cut the hole the size of the box that projects out from the building +1/8" . I use a 4 1/2 inch 90 degree grinder. Then you pop the siding loose in the area of the hole so you can fit the nailing flange in behind the siding and nail it. Renail the siding and pop on the trim ring which covers the 1/8" gap and you're done. Don't drive the nails in tight. The holes are slotted so the siding can grow and shrink with temperature swings. There a siding hook (about $5) that makes it easier to get the siding unhooked from the piece below it. Home despot has them and it looks kinda like a flat steel question mark with a hook on one end. You could probably do the job witout it if the siding isn't stretched too tight. I have one but I'm in Ct.